1. Field of the Invention
The present invention relates to cellular communication systems, and more particularly relates to a cellular communication system wherein a base station (also known as a gateway) of a cell can borrow channels for transmission and reception from adjacent cellular base stations.
2. Description of the Prior Art
Since the demand for channels to serve users of cellular communication systems is expected to grow rapidly in the near future, there is a strong interest to develop techniques which enhance system capacity for a given grade of service. Additionally, it is desirable for mobile communication systems to be able to accommodate spatially localized user communication overloads which do not normally occur within a particular cellular region. Such overloads (commonly referred to as "hot spots") may be caused by events such as vehicular traffic jams. Hot spot overloads can cause unacceptable degradation of performance and capacity for traditional cellular communication systems. These hot spots can be particularly disturbing to persons who desire to use the system, who for example, may be caught in highway traffic.
In order to reduce the likelihood that hot spots will overload a region of the cellular communication system, some schemes reserve a group of channels for use only when a hot spot develops. However, this is generally inefficient because the reserved channels are not utilized during normal system operation which results in a diminished normal system capacity. An alternative to the above involves borrowing communication channels for use in the "hot spot" cell from gateways which are not in an overload condition.
There are generally two types of channel borrowing techniques. The first involves a scheme wherein a channel (designated a borrowed channel) is temporarily transferred to a borrowing cell and used therein in a manner similar to that of channels which are permanently assigned to the borrowing cell. In addition, the borrowed channel is used with the same power as the channels permanently assigned for use by the borrowing cell. Dynamic channel assignment (DCA) and hybrid channel assignment (HCA) are two methods that utilize this channel borrowing scheme. In DCA, there is a central pool of channels which are accessed by each cell of the system. Each cell can temporarily borrow a channel from the central pool of channels for the duration of a transmission. When the transmission is complete, the channel that was borrowed by the cell is returned to the central pool of channels. In HCA, the channels to be used by the system are divided into two groups. Some channels are permanently assigned to specific cell gateways as in fixed channel assignment (FCA) schemes, and others are kept in a central pool for borrowing during regional overloads as in DCA. In HCA and DCA, channel locking is used to prevent an increase in co-channel interference which occurs when proximately located cells utilize the same channel for transmission. Channel locking prevents cellular gateways, within a minimum reuse distance, from simultaneously using the same channels for transmission or reception.
Channel locking, which is associated with channel borrowing, does have disadvantages. First, the number of channels that are available for lending to other cells is relatively limited. This is due to the fact that a channel can be borrowed by a cell only when the channel to be borrowed is not being used within the minimum channel reuse distance from the borrowing cell. Another disadvantage of channel locking is the difficulty in maintaining the required minimum co-channel reuse distance everywhere in the system. Because of this difficulty, DCA and HCA generally perform less satisfactorily under high communication traffic loads than FCA systems (wherein each cell is allocated a set of channels that are also used within other cells that are sufficiently distant from common channel cells so that co-channel interference will not pose a significant problem).
Several modified DCA and HCA schemes have been suggested to mitigate the need for channel locking. Systems have been proposed wherein a rearrangement of channels occurs when a channel becomes available in a cell that has borrowed a channel from another cell. This minimizes the traffic carried on borrowed channels. Systems have also been proposed wherein directional channel locking and locally optimized dynamic channel assignment are used to increase the number of channels available for borrowing and to minimize the channel reuse distance of borrowed channels. This provides a reduced probability of blocking incoming and outgoing transmissions. Since all of these systems still use channel locking, each is incapable of completely overcoming the associated inherent disadvantages.
In addition to the drawbacks of channel locking, further disadvantages of channel borrowing relates to the physical complexity of the system employing such a method. Specifically, the base station of each cell (i.e., gateway) must be able to transmit and receive not only on the channels specifically allocated to that cell, but also on any of the channels that are allocated to the central pool of channels. Finally, to implement channel borrowing at a given gateway, information regarding the channels being used at all cellular gateways within the channel reuse distance of the given cellular gateway must be known. This causes additional complexity in the management of system channel resources.
The second type of channel borrowing system involves assigning channels to each cellular gateway as in FCA. But, if a new call finds all channels assigned to the subject cell occupied, the call will not necessarily be blocked. Instead, if the user is also within range of a neighboring cell's gateway, the user will try to use a channel that was assigned to the neighboring cell's gateway. The link is established through the gateway of the neighboring cell. This type of borrowing scheme utilizes the overlapping coverage areas of cellular gateways. With this type of borrowing system, channels are not temporarily transferred from one cellular gateway to another, only the right to use a particular channel is transferred to (a user in) another cell.
An advantage of this type of channel borrowing is that each cellular gateway must only accommodate the channels assigned to the gateway itself. The simplicity of such an arrangement has accompanying limitations. Users of borrowed channels must be in the region of coverage overlap provided by the adjacent cell gateway. As a result, the users tend to be relatively far from the cellular gateways through which they transmit and receive signals. Therefore, the quality of borrowed channel transmissions is lower than that of a cell's regular assigned channels. By utilizing such a channel borrowing system, co-channel interference is also increased because the borrowed channels are used beyond their normal transmission range. In order to limit co-channel interference, the overlap among cells should be minimized. Overlap among two or three adjacent cells is usual, and therefore a given user can only access one or two neighboring additional cellular gateways. This limits the number of channels that are potentially available for transmission during a "hot spot" condition.